A flexible tank can be used for the transportation of non-hazardous bulk liquids or semi-liquid materials inside of shipping containers, such as the standardized containers used in multimodal international shipping. Standardized shipping containers all have the same width and are typically 20 feet, 40 feet or 53 feet in length, with two doors at one end thereof to insert and remove cargo. When a flexible tank is used in a standard shipping container, it typically extends the entire length of the container and contains a large amount of material. Volumetrically, flexible tanks can range from several thousand liters up to 26,000 liters depending on the bulk density of the liquid. There may be heaters and insulation when food grade products or other products having temperature range are shipped. A sturdy bulkhead, such as one made of steel, is placed at the end of the container with the doors to keep the flexible tank in place when the doors are opened.
Typically, a flexible tank has multiple (such as 2-4) polyethylene (PE) inner layers. The material is preferably a Low Linear Density Polyethylene (LLDPE), and the inner layers may or may not have a variety of thicknesses. Each inner layer of the tank is heat sealed together leaving a “coupon” on each end to which an outer strengthening layer or shell of woven polypropylene (WPP) with a weight between 180 g/m2 to 210 g/m2 is stitched using an Overlock stitch, normally referred to as butt-seaming. Single layer Low Linear Density Polyethylene (LLDPE) flexible tanks are also known. They on average have a standard thickness of 1000 microns and do not require WPP outer strengthening layer. Whether it is a single layer or a multi-layer tank, both the inner and outer layers start out as tubular material.
A load/discharge valve is normally positioned at the bottom rear end of the tank facing through the bulkhead and is supported by a molded “gland” that is heat sealed into the body of the flexible tank. The gland is manufactured with a center threaded collar into which the valve is screwed completely and mechanically secured. The discharge valve and the hole in the bulkhead providing access to the discharge valve are located opposite the approximate center of a door of the container so that only that one door has to be opened to access the discharge valve with a pump to fill or remove the material. The flexible tank can be used to transport non-hazardous chemicals such as but not limited to base oils, castor oil, motor oil, paints and latex. With an appropriate inner liner, the flexible tank can be used to transport a variety of bulk liquid food grade products, including but not limited to, juice, juice concentrate, beer, wine, sucrose, glucose, glycerin, water, corn oil and other vegetable oils, choline chloride, honey, corn syrup or molasses. Typically, a flexible tank is used only once in sterile condition and is disposed of rather than cleaned.
The complete discharge of a flexible tank can be somewhat difficult. The inner layers may collapse trapping materials in pockets or folds at a considerable distance from the discharge valve and towards the end of the discharge process, the pump tends to suck the inner layers into the valve in any event. People may enter the container and attempt to maneuver the tank in such a way that will push the remaining product to be funneled to the discharge valve. This involves picking up the flexible tank with laden product, folding or rolling it up, and in some cases using brooms by starting at the rear of the tank and working the product forward to the discharge valve. At the same time, the pump has to be manipulated to reduce the possibility of the polyethylene layer of the flexible tank from being sucked into the discharge hose by the negative pressure the pump is producing while pumping the product out of the tank. This is a time consuming operation and normally requires at least two people. There can also be safety concerns for the people located inside the container with a flexible tank that has been only partially emptied.
When handling more viscous products, such as orange juice concentrate, the discharge process becomes more difficult and will leave substantially more residual product as opposed to a free flowing product such as water. Methods such as introducing water into the tank to dilute and liquefy the residual orange juice concentrate or other product have proven to be unsuitable. A water rinse is difficult to perform inside the flexible tank. And the process to collect the diluted residue in drums and run the residue through an evaporator to remove the water (returning the product to concentrate) is cumbersome and expensive.